In a manufacturing process of a semiconductor device, a fine pattern is generally formed using a photolithographic method. In the formation of the fine pattern, multiple substrates, which are referred to as transfer masks, are usually used. The transfer mask is formed by providing the fine pattern comprised of a metal thin film, etc. on a generally transparent glass substrate. The photolithographic method is also used in the manufacture of the transfer mask.
Refinement of a pattern for the semiconductor device requires the refinement of a mask pattern formed in the transfer mask as well as shortening of a wavelength of an exposure light source used in photolithography. Recently, the exposure light sources used in the manufacture of semiconductor devices are shifting from KrF excimer lasers (wavelength: 248 nm) to ArF excimer lasers (wavelength: 193 nm), that is, shorter wavelength light sources are increasingly used.
The known types of transfer masks include a binary mask including a light shielding film pattern made of a chromium-based material on a conventional transparent substrate, as well as a half tone phase shift mask. The half tone phase shift mask comprises a light semitransmissive film pattern on the transparent substrate. The light semitransmissive film (half tone phase shift film) has functions for allowing transmission of light at an intensity not substantially contributing to the light exposure and for providing the light transmitted through the light semitransmissive film with a predetermined phase difference with respect to light traveling the same distance through air, thereby generating a so-called phase shift effect.
Generally, in the transfer mask, a periphery region outside the region in which a transfer pattern is formed should ensure optical density (OD) not less than a predetermined value such that, upon the exposure transfer to a resist film on a semiconductor wafer using an exposure apparatus, the resist film will not be affected by the exposure light transmitted through the periphery region. Usually, in the periphery region of the transfer mask, OD is desirably 3 or more, and at least about 2.8 of OD is required. However, the light semitransmissive film of the half tone phase shift mask has a function for allowing the transmission of the exposure light at a predetermined transmittance, and thus, it is difficult to ensure the optical density required for the periphery region of the transfer mask by this light semitransmissive film alone. Therefore, as with the phase shift mask blank disclosed in Patent Document 1, a light shielding film (light blocking film) is laminated onto a semitransparent film having predetermined phase shift amount and transmittance with respect to the exposure light, so that a laminated structure of the semitransparent film and light shielding film ensures the predetermined optical density.
On the other hand, there is a phase shift mask blank in which a light shielding film provided on a phase shift film is formed from a material containing transition metal and silicon, as disclosed in Patent Document 2. In this phase shift mask blank, the material containing transition metal and silicon is also used as a material forming the phase shift film as with the conventional case. Thus, it is difficult to ensure etch selectivity between the phase shift film and light shielding film in the dry etching. Therefore, the phase shift mask blank of Patent Document 2 comprises, between the phase shift film and light shielding film, an etching stopper film made of a material containing chromium. Also, it comprises, on the light shielding film, an etching mask film made of the material containing chromium.